Semiconductor device having floating body type transistor

ABSTRACT

A semiconductor device comprises a floating body type transistor and first and second circuits. The transistor has a floating body and a source-drain path inserted between first and second circuit nodes. The first circuit supplies a first signal to the gate of the transistor, and the first signal changes between a first logic level that holds the transistor in a non-conductive state and a second logic level that directs the transistor into a conductive state. The second circuit supplies a first voltage level near the second logic level to the first circuit node and supplies a second voltage level near the second logic level to the second circuit node, each as a level in a state where the transistor is not utilized. Thereby the gate capacitance of the transistor can be kept small as viewed from the gate, and high-speed operation and a reduction in consumption current can be achieved.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device having aconfiguration in which a floating body type transistor is used as a pathgate.

2. Description of Related Art

Conventionally, a planer type MOS transistor has been generally used asa transistor structure of a semiconductor device. Meanwhile, attentionhas recently been focused on a floating body type transistor as atransistor structure for achieving higher integration of thesemiconductor device, in which a body between source and drain formedover a substrate via an insulating film operates in a floating state.For example, a transistor having a device structure such as SOI (Siliconon Insulator) structure, Fin-FET structure or pillar-shaped structurehas been proposed as the floating body type transistor (for example,refer to Patent Reference 1).

-   [Patent Reference 1] Japanese Patent Application Laid-open No.    2003-68877 (U.S. Pat. No. 6,621,725)

In order to achieve high-speed operation by supplying a signal to a gateof a transistor used in the semiconductor device, a gate capacitance ofthe transistor is desired to be small as long as possible. However, thegate capacitance of the above-mentioned planer type transistor reachesthe bottom when gate and source voltages are approximately equal to eachother, and the transistor has C-V characteristics (relation between agate-source voltage and the gate capacitance) with which the gatecapacitance is increased even if the gate voltage changes upward ordownward from this point. In other words, if the gate voltage is higherthan the source voltage, a capacitance between a gate wiring and aninversion layer becomes dominant as viewed from the gate wiring, and ifthe gate voltage is lower than the source voltage, a capacitance betweenthe gate wiring and a substrate becomes dominant as viewed from the gatewiring, in both of which the gate capacitance increases. In these cases,reducing the gate capacitance by controlling the gate voltage based on arelation with the source voltage is not so effective, which poses aproblem that an effective control is difficult since the position of theabove bottom fluctuates due to variations in manufacturing process,operation voltages and operating temperature. For example, when using atransistor as a path gate inserted in a signal path in the semiconductordevice, there is a problem that proper control cannot be achieved interms of reducing the gate capacitance and thus high-speed operation ishindered. There is the same problem when using the transistor in a logiccircuit that requires the high-speed operation. Further, even whenemploying the above floating body type transistor in the semiconductordevice, it is difficult to achieve the reduction in gate capacitance bythe above-mentioned conventional method of the gate voltage.

SUMMARY

One of aspects of the invention is a semiconductor device comprising: atransistor having a gate, a source coupled to one of first and secondnodes, a drain coupled to the other of the first and second nodes, and abody between the source and drain, the body being brought into anelectrically floating state; a first circuit supplying the gate of thetransistor with a first signal changing between a first logic level thatholds the transistor in a non-conductive state and a second logic levelthat directs the transistor into a conductive state; and a secondcircuit supplying, when the transistor is not utilized, a first voltagelevel near the second logic level to the first circuit node and a secondvoltage level near the second logic level to the second circuit node.

According to the semiconductor device of the invention, the floatingbody type transistor is arranged as a path gate of a signal path, forexample, and when the first signal supplied to the gate is shifted fromthe first logic level to the second logic level in a circuit state wherethe transistor is not utilized, a small gate capacitance of thetransistor can be maintained as viewed from a gate wiring byappropriately controlling the relation of voltages of the logic levelsand the first and second circuit nodes in the operation. Thus, awaveform of the first signal is not rounded so as to enable high-speedcontrol, and it is possible to achieve high-speed operation in a circuithaving the floating body type transistor as the path gate and areduction in consumption current.

The present invention can be applied to various circuits. For example,the present invention can be applied to a configuration including bitlines, sense amplifiers connected to the bit lines, and a firstinput/output line connected to the sense amplifiers. In this case, bycontrolling the voltages in the above manner using the floating bodytype transistor that is inserted between an output node of each senseamplifier and the first input/output line, it is possible to achieve anincrease in speed of a read operation and a reduction in consumptioncurrent.

According to the present invention, since the relation of voltages ofthe gate and the source/drain is appropriately controlled using thefloating body type transistor, the gate capacitance as viewed from thegate wiring can be kept small, and it is possible to achievehigher-speed circuit operation and a reduction in consumption current.Particularly, in a configuration in which the first signal selectivelycontrols a large number of transistors, influence of the gatecapacitance of a non-selected transistor becomes larger, and therefore alarge effect can be obtained by employing the voltage control using thefloating body type transistor.

BRIEF DESCRIPTION OF THE DRAWINGS

The above features and advantages of the present invention will be moreapparent from the following description of certain preferred embodimentstaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram showing an outline configuration of a memorycell array and column circuits in a DRAM of a first embodiment;

FIG. 2 is a diagram showing a specific circuit configuration example ofa part in the column circuits of FIG. 1;

FIG. 3 is a diagram showing a specific circuit configuration example ofa sense amplifier of FIG. 2.

FIG. 4 is a diagram explaining a C-V characteristic in using a floatingbody type MOS transistor in the first embodiment;

FIG. 5 is a diagram showing a configuration example of a general 3-to-8selector;

FIG. 6 is a diagram showing a configuration example of a 3-to-8 selectorto which the invention is applied;

FIGS. 7A and 7B are diagrams explaining a circuit configuration exampleof each logic circuit included in the circuit configuration of FIG. 6;

FIG. 8 is a diagram showing a modification of the logic circuit of FIG.7A;

FIG. 9 is diagram showing a structural example of a MOS transistor usingSOI structure;

FIG. 10 is a diagram showing a structural example of a MOS transistorusing Fin-FET structure; and

FIG. 11 is a diagram showing a structural example of a MOS transistorusing pillar-shaped structure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will be now described herein with reference toillustrative embodiments. Those skilled in the art will recognize thatmany alternative embodiments can be accomplished using the teachings ofthe present invention and that the invention is not limited to theembodiments illustrated for explanatory purposes.

First Embodiment

A first embodiment describes an example in which the present inventionis applied to column circuits of a DRAM (Dynamic Random Access Memory)as the semiconductor device. FIG. 1 is a block diagram showing anoutline configuration of a memory cell array and column circuits in theDRAM of the first embodiment. In the block diagram of FIG. 1, a memorycell array

-   -   and a sense amplifier array 11 attached to the memory cell array        10 form a unit area, and a plurality of unit areas are aligned        in a bit line direction. In the memory cell array 10, a        plurality of word lines WL and a plurality of bit lines BL        orthogonal to the word lines WL are arranged, and a plurality of        memory cells MC are formed at intersections thereof. Each bit        line BL is connected to a sense amplifier of the sense amplifier        array 11. In the memory cell array 10, a signal is read out from        the memory cell MC selected by the word line WL to the bit line        BL, and a corresponding sense amplifier senses and amplifies the        signal of the bit line BL and latches it.

At one end of a row of the plurality of unit areas, there is arranged acolumn decoder 14 (the first circuit of the invention) that selectivelyactivates a plurality of column selection signals YS (the first signalof the invention) in response to a column address. In the example ofFIG. 1, each of n+1 column selection signal YS (YS0 to YSn) is suppliedto each of n+1 column selection circuits 12 adjacent to each senseamplifier array 11, and connections between four sense amplifiers of thesense amplifier array 11 and four local input/output lines LIO (LIO0,LIO1, LIO2 and LIO3) are controlled in response to the column selectionsignals YS. Since each column selection signal YS is supplied to aplurality of sense amplifier arrays 11, not only sense amplifiers of aselected sense amplifier array 11, but also sense amplifiers ofnon-selected sense amplifier arrays 11 are connected to the localinput/output lines LIO (LIO0 to LIO3) at the same time.

Each switch circuit 13 controls connections between the four localinput/output lines LIO (LIO0 to LIO3) and the four main input/outputlines MIO (MIO0 to MIO3). As shown in FIG. 1, the local input/outputlines LIO in the plurality of unit areas are connected to common maininput/output lines MIO through a plurality of switch circuits 13. Thesignal transmitted through the main input/output lines MIO is outputtedto outside via a read amplifier (not shown).

Next, a specific circuit configuration of a part in the column circuitsof FIG. 1 will be described with reference to FIGS. 2 and 3. FIG. 2shows one sense amplifier SA of the sense amplifier array 11 and acircuit portion associated with the local input/output line LIO and themain input/output line MIO connected to the sense amplifier in the blockdiagram shown in FIG. 1. Further, FIG. 3 shows a specific circuitconfiguration example of the sense amplifier SA of FIG. 2.

The sense amplifier SA has a single-ended circuit configurationincluding three P-channel type transistors Q20, Q23 and Q25 and sevenN-channel type transistors Q21, Q22, Q24, Q26, Q27, Q28 and Q29, asshown in FIG. 3. The transistor Q20 precharges the bit line BL to apower supply voltage VARY in response to a control signal PCB applied toits gate. The transistor Q21 controls a connection between the bit lineBL and a node N1 in response to a control signal LTC applied to itsgate. The transistor Q22 controls a connection between the bit line BLand a node N2 in response to a control signal RES applied to its gate.

The transistors Q23, Q24, Q25 and Q26 form a latch circuit, whichdetermines a signal voltage of the bit line BL in a binary value andlatches it. A pair of transistors Q23 and Q24 forms an inverter whoseinput is the node N1, a pair of transistors Q25 and Q26 forms aninverter whose input is the node N2, and these two inverters arecross-coupled to each other at their inputs and outputs. The transistorQ27 for a write operation is connected between the node N1 and an outputnode NS, and a control signal WEB is applied to its gate. Twotransistors Q28 and Q29 for a read operation are connected in seriesbetween the output node NS and a ground potential VSS. The node N2 isconnected to the gate of the transistor Q28, and a control signal RE isapplied to the gate of the transistor Q29.

When the sense amplifier SA shown in FIG. 3 is not selected, the powersupply voltage VARY is supplied to the output node NS by controlling thecontrol signals PCB, RES and RE to be “low” respectively and controllingthe control signals WEB and LTC to be “high” respectively. That is, thepower supply voltage VARY is supplied to the bit line BL by thetransistor Q20 and thereafter the power supply voltage VARY is alsosupplied to the output node NS through the transistors Q21 and Q27 fromthe bit line BL.

Returning to FIG. 2, an N-channel type transistor Q10 as the floatingbody type transistor of the invention is a unit switch included in thecolumn selection circuit 12 of FIG. 1 and is connected between theoutput node NS (the first circuit node the invention) at the output sideof the sense amplifier SA and the local input/output line LIO (thesecond circuit node of the invention). The column selection signal YS isapplied to the gate of the transistor Q10, which becomes conductive whenthe column selection signal YS is “high” and becomes non-conductive whenthe column selection signal YS is “low”. For example, the high level(the second logic level of the invention) of the column selection signalis set to a power supply voltage VDD, and the low level (the first logiclevel of the invention) thereof is set to the ground potential VSS. Inthe first embodiment, the floating body type MOS transistor is employedas the transistor Q10 that is a column selection switch, therebyimproving operation characteristics based on a relation between voltagesof the transistor Q10 and the column selection signal YS, which will bedescribed in detail later.

A P-channel type transistor Q11 functions as a precharge circuitprecharging the local input/output line LIO to the power supply voltageVARY in response to a control signal PCL applied to its gate. AnN-channel type transistor Q12 is a unit switch included in the switchcircuit 13 of FIG. 1, and is connected between the local input/outputline LIO and the main input/output line MIO. Switching of the transistorQ12 is controlled in response to a control signal LS applied to itsgate. A P-channel type transistor Q13 precharges the main input/outputline MIO to the power supply voltage VARY in response to a controlsignal PCM applied to its gate. In addition, the sense amplifier SA andthe transistor Q11 integrally function as the second circuit of theinvention.

In FIG. 2, when the column selection signal YS is activated to “high”,an output signal of the sense amplifier SA is coupled to the localinput/output line LIO via the transistor Q10, and further when thecontrol signal LS is activated, the local input/output line LIO isconnected to the main input/output line MIO via the transistor Q12. In aprecharge operation, both the control signals PCL and PCM are changed to“low” so that both the local input/output line LIO and the maininput/output line MIO go into a state of being precharged to the powersupply voltage VARY.

In the example of FIG. 2, although the power supply voltage VARY issupplied to the sense amplifier SA and the transistors Q11 and Q13 forprecharging, respectively, a power supply voltage level can be properlychanged. However, in a non-selected sense amplifier SA corresponding tothe activated column selection signal YS, it is preferable to drive thesense amplifier SA and the transistor Q11 with power supply voltageshaving the same level for the purpose of preventing a current fromflowing between a non-selected bit line BL and a non-selected localinput/output line LIO.

Although the floating body type MOS transistor is employed as thetransistor Q10 in FIG. 2, other transistors Q11 to Q13 and Q20 to Q29are not restricted, for which the floating body type MOS transistor orother kinds of transistors may be used.

Next, C-V characteristics (relation between a gate-source voltage and agate capacitance) in using the floating body type MOS transistor in thefirst embodiment will be described with reference to FIG. 4. FIG. 4 is agraph showing the C-V characteristics of the floating body typetransistor Q10 of FIG. 2. In FIG. 4, another C-V characteristic obtainedby replacing the transistor Q10 with a conventional planer type MOStransistor is shown for comparison, which is overlapped with a C-Vcharacteristic of the floating body type transistor Q10. In addition,conditions of a source voltage Vs, a drain voltage Vd, the power supplyvoltages VDD, VARY, and a bit line voltage VBLP are Vs=Vd=VARY (=1.0V)for the floating body type MOS transistor, and are Vs=Vd=VBLP (=0.5V)for the planer type MOS transistor, assuming VDD=1.3V for both cases.

In FIG. 4, a threshold voltage Vt (=0.3V) is at the center of agate-source voltage Vgs (hereinafter, referred to simply as “Vgs”) alonga horizontal axis. As shown in FIG. 4, in a region where Vgs exceeds thethreshold voltage Vt, respective gate capacitances of the floating bodytype MOS transistor and the planer type MOS transistor are approximatelyequal to each other, and as Vgs increases, the gate capacitances rapidlyincrease until reaching a predetermined level. This is because acapacitance between the gate and an inversion layer becomes dominant ineach of the gate capacitances of the floating body type MOS transistorand the planer type MOS transistor in the region where Vgs exceeds thethreshold voltage Vt.

In contrast, in a region where Vgs is lower than the threshold voltageVt, the respective gate capacitances of the floating body type MOStransistor and the planer type MOS transistor change differently fromeach other. That is, as shown in FIG. 4, in a region R1 where Vgs islower than the threshold voltage Vt in the floating body type MOStransistor, a capacitance between the gate and a substrate is invisiblesince a body between the source and drain is in a floating state, sothat the gate capacitance is approximately 0. Meanwhile, in the planertype MOS transistor, the gate capacitance decreases in a center regionwhere Vgs is near the threshold voltage Vt, and the gate capacitanceincreases in a region where Vgs decreases relatively to the centerregion since influence of the capacitance between the gate and thesubstrate becomes larger.

Considering a transition of Vgs corresponding to the control of thecolumn selection signal YS applied to a gate electrode when thetransistor Q10 is changed from a non-conductive state into a conductivestate, FIG. 4 shows a transition Sa for the floating body type MOStransistor and a transition Sb for the planer type MOS transistor. Thatis, when the column selection signal YS is activated from the groundpotential VSS to the power supply voltage VDD (=1.3V), Vgs changes from−1V to +0.3V in the transition Sa since Vs=Vd=VARY (=1V) is maintained,and Vgs changes from −0.5V to +0.8V in the transition Sb sinceVs=Vd=VBLP (=0.5V) is maintained. The gate capacitance of the floatingbody type MOS transistor is maintained at 0 within a range of thetransition Sa, and the gate capacitance of the planer type MOStransistor largely changes within a range of the transition Sb.

According to the C-V characteristics of FIG. 4, in order to direct thetransistor Q10 into the conductive state, the column selection signal YSis activated in the circuit configuration of FIG. 2, and when the columnselection signal YS changes from “low” to “high”, the gate capacitanceis maintained at approximately 0. Thus, the gate capacitance as viewedfrom a line of the column selection signal YS decreases at this point,there is an effect of obtaining a high-speed waveform being not rounded,and consumption current in a column selection operation can be reduced.As described above, one line of the column selection signal YS isconnected to gates of a large number of transistors Q10, and thereforeif the column selection operation is performed in a state where thepower supply voltage VARY is supplied to sources/drains of thetransistors Q10 corresponding to non-selected sense amplifiers SA, theeffect of reducing the consumption current correspondingly increases.

Additionally, the first embodiment has described a case of using an NMOStype transistor as the floating body type transistor Q10. However, theinvention can be also applied to a case of using a PMOS type transistor.In this case, a relative voltage relation of the gate and thesource/drain of the transistor may be inverted relative to the case ofthe first embodiment.

Second Embodiment

A second embodiment describes an example in which the present inventionis applied to a general logic circuit in the semiconductor device.Hereinafter, as one example of the logic circuit, a 3-to-8 selectorselecting one of eight output signals based on three input signals willbe described with reference to FIGS. 5 to 8. FIG. 5 shows aconfiguration example of a general 3-to-8 selector 20 for comparison,and FIG. 6 shows a configuration example of a 3-to-8 selector 21 towhich the invention is applied.

First, the 3-to-8 selector 20 of FIG. 5 selects one of eight outputsignals OUT0 to OUT7 in accordance with a logical combination of threeinput signals INT1, INT2 and INT3 so that the selected signal changes toa high level, and the 3-to-8 selector 20 includes three inverters on aninput-side, eight 3-input NAND gates, and eight inverters on anoutput-side. Each of the 3-input NAND gates outputs a low level when allinput three signals are at a high level, and the output thereof becomesan output signal OUTi (i=0 to 7) of the high level via each inverter.

Meanwhile, the 3-to-8 selector 21 of FIG. 6 is configured by replacingthe eight 3-input NAND gates in the circuit configuration of FIG. 5 witheight logic circuits 30, and each of the logic circuits 30 includes thefloating body type MOS transistor having the C-V characteristics of FIG.4. In addition, a basic operation of the 3-to-8 selector 21 of FIG. 6 iscommon to that of the 3-to-8 selector 20 of FIG. 5.

FIG. 7A shows a circuit configuration example of each logic circuit 30included in the circuit configuration of FIG. 6. The logic circuit 30includes N-channel type transistors Q30, Q31 and P-channel typetransistors Q32, Q33, and outputs a signal corresponding to a logicalcombination of three input signals S1, S2 and S3 to a node Ni. In thelogic circuit 30, the floating body type MOS transistor is used for eachof the transistors Q30 and Q31. The signals S1, S2 and S3 change inaccordance with a logical state of the input signals INT1, INT2 and INT3of FIG. 6. As shown in a truth table of FIG. 7B, in the configurationexample of the logic circuit 30 of FIG. 7A, a selected state appearswhen the signals S1 and S2 are “high” (the power supply voltage VDD) andthe signal S3 is “low” (the ground potential VSS), and thus the node Nichanges to “low” so that the output signal OUTi becomes “high” via theinverter. In other conditions, the output signal OUTi becomes “low”.

In FIG. 7A, the signal S1 is inputted to gates of a pair of transistorsQ30 and Q32 that form an inverter, and an output side of this inverteris connected to the node Ni. The signal S2 is inputted to the gate ofthe transistor Q33 connected between the power supply voltage VDD andthe node Ni. The signal S2 is inputted to the gate of the transistor Q31connected in series with the transistor Q30, and the signal S3 isinputted to the source of the transistor Q31. When the signals S1 and S2are “high” and the signal S3 is “low”, the transistors Q30 and Q31become conductive so as to decrease the potential of the node Ni to“low” (selected state). Meanwhile, when the signal S3 is “high”, thenode Ni remains “high” (non-selected state) since no current flowsthrough the transistors Q30 and Q31.

Here, in an operation of the non-selected state where the signal S3 is“high”, the power supply voltage VDD is supplied to sources of thefloating body type transistors Q30 and Q31. Therefore, when the signalsS1 and S2 change from “low” to “high”, or when the signals S1 and S2change from “high” to “low”, the transistors Q30 and Q31 transit in theregion where the C-V characteristics of FIG. 4 is low (region of Vgs<0).Accordingly, gate capacitances of the transistors Q30 and Q31 as viewedfrom lines of the signals S1 and S2 maintain a value near 0, andtherefore it is possible to achieve high-speed operation waveforms and areduction in consumption current.

FIG. 8 shows a circuit configuration example of a logic circuit 30 athat is a modification of the logic circuit 30 of FIG. 7A. Most parts inthe logic circuit 30 a of FIG. 8 are common to those in the logiccircuit 30 of FIG. 7. However, a difference exists in that a P-channeltype transistor Q34 is provided in addition to the above transistors Q30to Q33. The transistor Q34 is connected between the power supply voltageVDD and the node Ni, and the output signal OUTi is applied to its gate.Thereby, when the output signal OUTi changes to “low”, the transistorQ34 turns on so as to supply the power supply voltage VDD to the nodeNi, and therefore it is possible to prevent the node Ni from being in afloating state.

[Device Structure]

In the following, device structures of the floating body type transistorof the invention will be described with reference to FIGS. 9 to 11. FIG.9 shows a structural example of a MOS transistor using SOI (Silicon onInsulator) structure. In the structural example of FIG. 9, an insulatingfilm 101 is formed on a silicon substrate 100, and, for example, N typesource-drain diffusion layers 102 and 103 are formed on both sides onthe insulating film 101. For example, a P-type body region 104 is formedin a region between the source-drain diffusion layers 102 and 103. Agate electrode 106 is formed over the body region 104 via a gateinsulating film 105. As described above, the body region 104 iselectrically separated from the surrounding parts so as to be in thefloating state.

FIG. 10 is a perspective view showing a structural example of a MOStransistor using Fin-FET structure. In the structural example of FIG.10, an insulating film 201 is formed on a silicon substrate 200, and aso-called Fin between source/drain electrodes 202 and 203 on theinsulating film 201 functions as a body. A gate electrode 205 is formedover the Fin via a gate insulating film 204. The body under the gateelectrode 205 is electrically separated from the surrounding parts so asto be in the floating state.

FIG. 11 shows a structural example of a MOS transistor usingpillar-shaped structure. In the structural example of FIG. 11, forexample, N+ type source/drain regions 301 and 302 are formed in lowerand upper layers of a pillar-shaped region over a silicon substrate 300,and a body 303 as, for example, a P-type region is formed between thesource/drain regions 301 and 302. An interlayer insulating film 304surrounds the pillar-shaped region, and a gate electrode 306 surroundingthe body 303 via a gate insulating film 305 is formed inside theinterlayer insulating film 304. A wiring layer 307 used as, for example,a bit line is formed over the source/drain region 302. Also, in thisstructural example, the body 303 is electrically separated from thesurrounding parts so as to be in the floating state.

In the foregoing, the preferred embodiments of the present inventionhave been described. However the present invention is not limited to theabove embodiments and can variously be modified without departing theessentials of the present invention. That is, the present inventioncovers the various modifications which those skilled in the art cancarry out in accordance with all disclosures including claims andtechnical ideas.

The present invention can be applied to various semiconductor devicessuch as CPU (Central Processing Unit), MCU (Micro Control Unit), DSP(Digital Signal Processor), ASIC (Application Specific IntegratedCircuit), ASSP (Application Specific Standard Product) and the like, inaddition to the DRAM. Further, the present invention can be applied tovarious device structures such as SOC (System on Chip), MCP (Multi ChipPackage) and POP (Package on Package) and the like. Furthermore, varioustransistors can be used in the embodiments. For example, a field-effecttransistor (FET) can be used in the embodiments, and various types ofFETs such as MIS (Metal-Insulator Semiconductor), TFT (Thin FilmTransistor), and the like can be used in the embodiments.

The invention claimed is:
 1. A semiconductor device comprising: atransistor having a gate, a source coupled to one of first and secondcircuit nodes, a drain coupled to the other of the first and secondcircuit nodes, and a body between the source and drain, the body beingbrought into an electrically floating state; a first circuit supplyingthe gate of the transistor with a first signal changing between a firstlogic level that holds the transistor in a non-conductive state and asecond logic level that directs the transistor into a conductive state;and a second circuit supplying, when the transistor is not utilized, afirst voltage level near the second logic level to the first circuitnode and a second voltage level near the second logic level to thesecond circuit node.
 2. The semiconductor device as claimed in claim 1,wherein the first and second voltage levels are substantially equal toeach other.
 3. The semiconductor device as claimed in claim 1, whereinthe first and second voltage levels are different from each other. 4.The semiconductor device as claimed in claim 1, wherein a relationshipin voltage among the second logic level, the first voltage level and thesecond voltage level is set such that a voltage between the gate andsource of the transistor does not exceed a threshold voltage of thetransistor.
 5. The semiconductor device as claimed in claim 1, whereinthe second logic level is higher than the first logic level.
 6. Thesemiconductor device as claimed in claim 5, wherein the transistor is ofan NMOS type.
 7. The semiconductor device as claimed in claim 1, whereinthe second logic level is lower than the first logic level.
 8. Thesemiconductor device as claimed in claim 7, wherein the transistor is ofa PMOS type.
 9. The semiconductor device as claimed in claim 1, whereinthe transistor is configured as a part of a logic circuit performing alogical operation on first and second input signals, the first signalserving as the first input signal, and the second input signal beingsupplied to the first circuit node.
 10. The semiconductor device asclaimed in claim 9, wherein an additional transistor is provided betweenthe second circuit node and a voltage line supplied with the secondvoltage level, and the additional transistor becomes conductive when thefirst input signal is at the first logic level to supply the secondvoltage level to the second circuit node.
 11. A semiconductor devicecomprising: a bit line receiving a data signal from a selected memorycell; a sense amplifier connected to the bit line, the sense amplifieroutputting at an output node thereof a signal responsive to the datasignal in a data read operation and a first voltage level in a prechargeoperation; a first input/output line; a transistor having a source-drainpath inserted between the output node of the sense amplifier and thefirst input/output line, the transistor being of an electricallyfloating body in which a body between a source and drain is electricallyfloating state; a control circuit supplying a control signal to a gateof the transistor, the control signal changing between a first logiclevel that holds the transistor in a non-conductive state and a secondlogic level that directs the transistor into a conductive state; and aprecharge circuit supplying a second voltage level to the input/outputline in the precharge operation, wherein when the control circuitchanges the control signal from the first logic level to the secondlogic level in response to non-selection of the sense amplifier, thefirst voltage level supplied to the output node and the second voltagelevel supplied to the input/output line are maintained respectively at avalue near the second logic level.
 12. The semiconductor device asclaimed in claim 11 further comprising: a second input/output line; anda switch circuit controlling a connection state between the first andsecond input/output lines.
 13. The semiconductor device as claimed inclaim 11, wherein the sense amplifier is of a single-ended type andincludes a latch circuit in which a first inverter having a pair offirst transistor and a second inverter having a pair of secondtransistor are cross-coupled to each other at inputs and outputsthereof.